The present invention relates to a coated cutting tool insert designed to be used in metal machining. The substrate can be a cemented carbide, cermet, ceramic or cBN on which a hard and wear resistant coating is deposited. The coating exhibits an excellent adhesion to the substrate covering all functional parts thereof. The coating is composed of one or more refractory layers of which at least one layer is a strongly textured alpha-alumina (α-Al2O3) deposited in the temperature range of from about 750 to about 1000° C.
A crucial step in the deposition of different Al2O3 polymorphs is the nucleation step. κ-Al2O3 can be grown in a controlled way on {111} surfaces of TiN, Ti(C,N) or TiC having the fcc structure. TEM has confirmed the growth mode which is that of the close-packed (001) planes of κ-Al2O3 on the close-packed {111} planes of the cubic phase with the following epitaxial orientation relationships: (001)κ//(111)TiX; [100]κ//[112]TiX. An explanation and a model for the CVD growth of metastable κ-Al2O3 have proposed earlier (Y. Yoursdshahyan, C. Ruberto, M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist, Theoretical Structure Determination of a Complex Material: κ-Al2O3, J. Am. Ceram. Soc. 82(6)(1999)1365-1380).
When properly nucleated, κ-Al2O3 layers can be grown to a considerable thickness (>10 μm). The growth of even thicker layers of κ-Al2O3 can be ensured through re-nucleation on thin layers of, for example TiN, inserted in the growing κ-Al2O3 layer. When nucleation is ensured the κ→α transformation can be avoided during deposition by using a relatively low deposition temperature (<1000° C.). During metal cutting, the κ→α phase transformation has confirmed to occur resulting in flaking of the coating. In addition to this there are several other reasons why α-Al2O3 should be preferred in many metal cutting applications. As shown earlier α-Al2O3 exhibits better wear properties in cast iron (U.S. Pat. No. 5,137,774).
However, the stable α-Al2O3 phase has been found to be more difficult to be nucleated and grown at reasonable CVD temperatures than the metastable κ-Al2O3. It has been experimentally confirmed that α-Al2O3 can be nucleated, for example, on Ti2O3 surfaces, bonding layers of (Ti,Al)(C,O) or by controlling the oxidation potential using CO/CO2 mixtures as shown in U.S. Pat. No. 5,654,035. The bottom line in all these approaches is that nucleation must not take place on the 111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al2O3 is obtained.
It should also be noted that in the prior-art methods higher deposition temperatures (about 1000° C.) are usually used to deposit α-Al2O3. When the nucleation control is not complete, as is the case in many prior-art products, the produced α-Al2O3 layers have, at least partly, been formed as a result of the κ-Al2O3→α-Al2O3 phase transformation. This is especially the case when thick Al2O3 layers are considered. These kinds of α-Al2O3 layers are composed of larger grains with transformation cracks. These layers exhibit much lower mechanical strength and ductility than the α-Al2O3 layers that are composed of nucleated α-Al2O3. Consequently, there is a need to develop techniques to control the nucleation step of α-Al2O3.
The control of the α-Al2O3 polymorph in industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. No. 5,137,774. Later modifications of this patent have been used to deposit α-Al2O3 with preferred layer textures. In U.S. Pat. No. 5,654,035 an alumina layer textured in the (012) direction and in U.S. Pat. No. 5,980,988 in the (110) direction are disclosed. In U.S. Pat. No. 5,863,640 a preferred growth either along (012), or (104) or (110) is disclosed. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al2O3 along the (10(10)) direction. US20020155325A1 describes a method to obtain a strong (300) texture in α-Al2O3 using a texture modifying agent (ZrCl4). The prior-art processes discussed above use all high deposition temperatures of about 1000° C.
US 2004/0028951A1 describes a new state-of-the-art technique to achieve a pronounced (012) texture. The commercial success of this kind of product demonstrates the importance to refine the CVD process of α-Al2O3 towards fully controlled textures.
It is well established that the water gas shift reaction, in the absence of H2S or other dopants, is the critical rate-limiting step for Al2O3 formation, and to a great extent, controls the minimum temperature at which Al2O3 can be deposited. Further it is well established that the water-gas shift reaction is very sensitive for deposition pressure.
Extensive work has been done to deposit CVD Al2O3 at lower temperatures. Several Al2O3 layers using other than AlCl3—CO2—H2 system have been investigated, including AlCl3—CO—CO2, AlCl3—C2H5OH, AlCl3—N2O—H2, AlCl3—NH3—CO2, AlCl3—O2—H2O, AlCl3—O2—Ar, AlX3—CO2 (where X is Cl, Br, I), AlX3—CO2—H2 (where X is Cl, Br, I), AlBr3—NO—H2—N2 and AlBr3—NO—H2—N2. It is emphasised that these studies have been carried out without dopants (such as H2S) and the effect of the deposition pressure has not been elucidated.
It is worth noticing that none of these latter systems have been commercially successful. Consequently, to provide a CVD process for depositing Al2O3 layers at temperatures below those currently used on a commercial scale is therefore highly desirable.
U.S. Pat. No. 6,572,991 describes a method to deposit γ-Al2O3 at low deposition temperatures. This work clearly shows that it is possible to obtain Al2O3 layers in the medium temperature range from the AlCl3—CO2—H2 system. However, in this work it was not realised that nucleation surface controls the phase composition of Al2O3 and that deposition of α-Al2O3 is thus possible at lower deposition temperatures. In the prior-art, it was considered impossible to deposit α-Al2O3 at low temperatures and it was believed that γ-Al2O3 and κ-Al2O3 were the unavoidable low temperature phases.